Internal combustion engine

ABSTRACT

When viewed from the upper face of a cylinder head in which an intake port communicating with a combustion chamber of an internal combustion engine is formed, a starting point of the port is defined as a point of intersection of the streamline of the intake port and an inlet-side opening plane of the intake port as projected on a horizontal plane, and an end point is defined as a point of intersection of the streamline of the port and the center axis of an intake valve as projected on the horizontal plane. The starting point is located closer to the center of the chamber than a straight line that contains the end point and extends in a direction orthogonal to the axis of the crankshaft on the plane, and the streamline of the port projected on the horizontal plane is curved toward the center of the chamber, with respect to a straight line that contains the starting point and extends in a direction orthogonal to the axis of the crankshaft on the plane.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to an internal combustion engine, and inparticular to an internal combustion engine in which a whirling airflowformed in a combustion chamber can be intensified even when a valve liftof intake valves is in a small to middle range.

2. Description of the Related Art

Various intake port designs have been proposed for the purpose ofimproving the manner in which intake air flows into a combustionchamber. For example, it has been proposed in Japanese PatentApplication Publication No. 10-169453 (JP-A-10-169453) to provide adirect injection type internal combustion engine having intake portsthat are inclined inwards, i.e., toward each other, such that thedistance between two intake ports becomes gradually smaller as they getcloser to the combustion chamber. In the direct injection type enginethus constructed, two tumble flows from the two intake ports jointogether so that strong turbulence is formed at around the ignition plugwhile a fuel-air mixture having a rich air-fuel ratio is formed ataround the ignition plug, whereby the engine can operate in a lean-burnmode with improved reliability. It has also been proposed in JapanesePatent Application Publication No. 09-236043 (JP-A-09-236043) to providea cylinder head of an internal combustion engine having upright portsthat are curved in the axial direction of a camshaft so as to securespacing between the upright ports and inner pivots for swing arms. Inthe cylinder head of the engine in which the swing arms of the innerpivot type and the upright ports are both provided, the space availablefor formation of the upright ports is greatly restricted. Even in thiscase, deterioration of the intake air efficiency or other problems dueto the space restrictions can be minimized.

In the meantime, technologies for producing whirling airflows, such astumbles or swirls, in cylinders of the engine are known in the art. Bycausing the engine of this type to produce a whirling airflow having anincreased intensity, it may be possible to expand a lean-burn region inwhich the engine is operable in a lean-burn mode and improve the outputperformance. In this regard, the manner of introducing the intake airinto the cylinder is one of the important factors in producing ahigh-intensity whirling airflow. With this background, technologies forimproving the manner of introducing the intake air into the cylinderhave been proposed in Japanese Patent Application Publication No.07-279751 (JP-A-07-279751) and Japanese Utility Model ApplicationPublication No. 59-135335 (JP-U-59-135335).

In order to produce a high-intensity whirling airflow in the cylinder,it is necessary to design intake ports so as to achieve the optimumintake-port shape or arrangement that satisfies this requirement.However, the whirling airflow is intensified typically when the valvelift of the intake valves is mainly in a middle to high range. FIG. 39Aand FIG. 39B schematically show typical examples of intake ports 10X(10Xa and 10Xb in FIG. 39A) and 10Y (10Ya and 10Yb in FIG. 39B) alongwith a combustion chamber 54, intake valves 55 and exhaust valves 56. InFIG. 39A and FIG. 39B, the intake ports 10X and 10Y as viewed from theupper face of the cylinder head are projected on a horizontal plane, anda point of intersection of the streamline of each intake port and theplane of the inlet-side opening of the intake port, as projected on thehorizontal plane S, is defined as a starting point P1, while a point ofintersection of the streamline of the intake port and the center axis ofthe intake valve 55, as projected on the horizontal plane S, is definedas an end point P2. Here, the center axis of the intake valve 55 doesnot mean the axis of a stem of the intake valve, but means the axis thatpasses the center of an umbrella portion of the intake valve. Also inFIG. 39A and FIG. 39B, straight line L1 represents a straight line thatcontains the starting point P1 and extends in a direction orthogonal tothe axis of the crankshaft on the horizontal plane S, and straight lineL2 represents a straight line that contains the end point P2 and extendsin a direction orthogonal to the axis of the crankshaft on thehorizontal plane S.

In FIG. 39A, the intake ports 10X (10Xa and 10Xb) are illustrated whichare shaped such that a straight line that connects the starting point P1with the end point P2 is substantially identical with the streamline Fof the intake port projected on the horizontal plane S (which will besimply called “projected streamline of the intake port”), and such thatthe distance between the two intake ports 10Xa and 10Xb graduallyincreases in a direction from the starting point P1 to the end point P2.In FIG. 39B, the intake ports 10Y (10Ya and 10Yb) are illustrated whichare shaped such that the projected streamline F of the intake port isnot located closer to the center of the combustion chamber 54 than thestraight line L1, namely, the projected streamline F is not located onthe inner side of the straight line L1 (in other words, is located onthe outer side of the straight line L1). With the intake ports 10X and10Y thus shaped, the intake air flows uniformly from the entire area ofthe downstream-side opening of the intake port 10 into the combustionchamber 54, and therefore, a whirling airflow is not favorablyintensified when the valve lift of the intake valve is in a small tomiddle range.

When the valve lift of the intake valve is in a small to middle range,in particular, a valve stem portion, for example, of the intake valvebecomes a major obstacle to flow of the intake air since the mainstreamof intake air normally has no particular directional characteristics,namely, the mainstream is not caused to flow in any particulardirection. Thus, a whirling airflow formed in the combustion chamber, ifany, is not always intensified as desired when the valve lift is in asmall to middle range. Therefore, some room for improvements remains inthe degree of mixing of the fuel-air mixture required for reducingemissions, such as HC and CO, and lessening deterioration of the fueleconomy, frame propagation characteristics required for improvement ofcombustion during cold start or lean-burn operation, and the combustionspeed required for preventing knocking. Nevertheless, no particularguidelines or schemes have been presented for designing the intake portsso that the whirling airflow can be favorably intensified even in thesmall to middle range of the valve lift.

Another problem encountered when intake air flows into the cylinder isthat the intake air interferes with the stem of the intake valve and isthus split into streams, whereby an intended flow of intake air cannotbe formed in the combustion chamber. In this respect, when the intakevalve is lifted largely, a large amount of intake air is intenselyintroduced into the cylinder, and therefore, a high-intensity whirlingairflow is relatively easily produced in the cylinder even in thepresence of the above problem. However, it is difficult to produce awhirling airflow having a sufficiently high intensity solely from theflow of intake air at the time when the intake valve is lifted high. Itis thus necessary to improve the manner of introducing the intake airinto the cylinder when the valve lift of the intake valve is in a smallto middle range, so as to produce a whirling airflow having asufficiently high intensity.

With the background as described above, it has been proposed inJP-A-07-279751 as identified above to offset the opening of the intakeport, along with the intake valve, to the outer side of the combustionchamber, thereby to form a large quantity of intake airflow directed tothe middle of the combustion chamber and draw the intake air toward themiddle of the combustion chamber. According to the technology proposedin the above-identified publication, therefore, it may possible toimprove the manner of introducing the intake air into the cylinder whenthe valve lift of the intake valve is in a small to middle range. Withthis technology, however, the intake air is split into branch streams bythe stem of the intake valve, thus still leaving a large amount ofintake air that does not flow toward the middle of the combustionchamber, which makes it difficult to provide a whirling airflow having asufficiently high strength. Also, with the proposed technology, theintake air may be concentrated too much at around the middle of thecombustion chamber when the intake valve is in a middle- to high-liftregion, and the intake air flowing into the cylinder may hit against thewall of the cylinder at an excessively high velocity, which may resultin a reduction of the intensity of the whirling airflow produced in thecylinder.

SUMMARY OF THE INVENTION

The present invention was developed in view of the above-describedproblems. Thus, it is an object of the invention to provide an internalcombustion engine wherein intake ports are designed so that a whirlingairflow produced in a combustion chamber can be intensified even whenthe valve lift of intake valves is in a small to middle range, andwherein the intake air can be introduced into the cylinder in afavorable manner so as to produce a whirling airflow in the cylinder,from the time when the valve lift of the intake valves is in a small tomiddle range.

According to one aspect of the invention, there is provided an internalcombustion engine including an intake port that communicates with acombustion chamber, and an intake valve having an umbrella portion and astem connected at one end thereof to the umbrella portion, wherein theintake port has a starting point that is a first point of intersectionof a streamline of the intake port and an inlet-side opening plane ofthe intake port, and an end point that is a second point of intersectionof the streamline of the intake port and a center axis of the intakevalve, as viewed from an upper face of a cylinder head in which theintake port is formed, the first and second points of intersection beingprojected on a horizontal plane. In this internal combustion engine, thestreamline of the intake port projected on the horizontal plane iscurved toward a center of the combustion chamber so as to be at leastpartially located closer to the center of the combustion chamber than afirst straight line that contains the starting point and extends in adirection orthogonal to an axis of a crankshaft on the horizontal plane,and a second straight line that contains the end point and extends in adirection orthogonal to the axis of the crankshaft on the horizontalplane.

While the positional relationship between the starting point and the endpoint may vary widely in designing the intake port, the intake air drawninto the combustion chamber through the intake port flows outwards, orstraight, or inwards, depending upon the design of the intake port. Inthe internal combustion engine having the above-described intake portdesign, the intake air flowing into the combustion chamber is given anincreased directional characteristic due to the curved shape, so thatthe intake air is more likely to flow in a particular direction. Thus,even when the valve lift of the intake valve is in a small to middlerange, a whirling airflow produced in the combustion chamber can beintensified.

In the internal combustion engine as described above, the starting pointmay be located closer to the center of the combustion chamber than thesecond straight line.

When the intake port is designed so that the starting point and the endpoint are positioned as described above, the intake air that flowsthrough the intake port normally tends to be directed outwards as awhole when flowing into the combustion chamber, as shown in FIG. 39A. Inthe internal combustion engine having the intake port curved in themanner as described above, on the other hand, the mainstream of intakeair, which has been deflected by an airflow control valve, or the like,can be directed so as to flow through the inner side of a valve stemportion of the intake valve when flowing into the combustion chamber.Thus, the mainstream of intake air is prevented from interfering withthe valve stem portion. Accordingly, a whirling airflow produced in thecombustion chamber can be intensified even when the valve lift of theintake valve is in a small to middle range.

In the engine as described above, the starting point may lie on thesecond straight line.

When the intake port is designed so that the starting point and the endpoint are positioned as described above, the intake air that flowsthrough the intake port normally tends to flow straight into thecombustion chamber. In the internal combustion engine having the curvedintake-port design as described above, on the other hand, the mainstreamof intake air can be directed in the manner as described above, andtherefore, a whirling airflow produced in the combustion chamber can beintensified even when the valve lift of the intake valve is in a smallto middle range.

In the engine as described above, the starting point may not be locatedcloser to the center of the combustion chamber than the second straightline.

When the intake port is designed so that the starting point and the endpoint are positioned as described above, the intake air that flowsthrough the intake port normally tends to be directed inwards as a wholewhen flowing into the combustion chamber. In the internal combustionengine having the curved intake-port deign as described above, thedirectional characteristic of the intake air can be enhanced (i.e., theintake air is further likely to be directed inwards), and therefore, awhirling airflow produced in the combustion chamber can be intensifiedeven when the valve lift of the intake valve is in a small to middlerange.

Thus, in the internal combustion engine as described above, the whirlingairflow produced in the combustion chamber can be intensified even whenthe valve lift of the intake valve is in a small to middle range.

In a preferred embodiment of the invention, the intake valve is aspecific intake valve in which the stem is offset such that an innerpassage region located closer to the center of the combustion chamber,out of two intake-air passage regions on the opposite sides of a planethat contains a center axis of the stem, becomes larger, and such thatthe center axis of the stem does not contain a center of a bottom faceof the umbrella portion.

In the engine according to the preferred embodiment as described above,the intake air that is about to flow into the combustion chamber towardthe middle thereof is more likely to be prevented from interfering withthe stem of the intake valve, thereby to form an increased quantity ofintake-air flow toward the middle of the combustion chamber when thevalve lift of the intake valve is in a small to middle range. Thus, theintake air can be introduced into the cylinder in a favorable manner soas to form a whirling airflow in the cylinder, from the time when thevalve lift of the intake valve is in a small to middle range.

The preferred embodiment as described above is different from thetechnologies proposed in JP-A-07-279751 and JP-U-59-135335 as identifiedabove, in that the stem is offset in the manner as described above, inview of the object of the invention to improve the manner of flowing ofintake air from the time when the valve lift of the intake valve is in asmall to middle range, and the level of the necessity to accomplish theobject. In this respect, the stem of the intake valve is conventionallyformed, in view of the strength and the ease in machining, such that thecenter axis of the stem contains the center of the bottom face of theumbrella portion, and the stem extends in a direction perpendicular tothe bottom face. As compared with the case where the location of theintake valve is changed along with the position of the opening of theintake port so as to improve the manner of flowing of intake air withoutoffsetting the stem, for example, it is advantageous or preferable tooffset the stem in terms of the increased freedom in changes. In thisrespect, too, the manner in which the intake air flows into the cylindercan be more favorably improved in the engine as described above.

While the plane that contains the center axis of the stem is notnecessarily limited to a single plane provided that the plane can dividethe intake-air passage region into the inner side and outer side of thestem with respect to the combustion chamber, this plane is specified asa plane that divides the intake-air passage region into the inner sideand the outer side with respect to the combustion chamber so that themainstream of intake air that flows into the cylinder so as to produce awhirling airflow in the cylinder is mainly contained in the innerpassage region. In this respect, in the engine in which the intake portis formed so as to produce a tumble flow as a whirling airflow in thecylinder, if the plane that contains the center axis of the stem isfurther made parallel to the axis of the cylinder, the mainstream ofintake air is mainly contained in the inner passage region, andtherefore, the intake air that is about to flow into the cylinder towardthe middle of the combustion chamber is more likely to be prevented,with higher reliability, from interfering with the stem.

The above statement that “the inner passage region becomes larger” meansthat the inner passage region becomes larger as compared with the casewhere a conventional intake valve is provided in which the stem is notoffset as in the preferred embodiment. In this respect, where theconventional intake valve in which the stem is not offset is provided inthe engine in which the intake port is formed so as to produce a tumbleflow in the cylinder, the intake-air passage region is usually supposedto be substantially equally divided into an inner passage region and anouter passage region by a plane parallel to the axis of the cylinder. Inthe preferred embodiment, therefore, the stem of the specific intakevalve is offset such that, more specifically, the inner passage regionbecomes larger than the other region, i.e., the outer passage region.

The stem of the specific intake valve as indicated above may be offsetto an upstream side with respect to the center of the specific intakevalve, in a direction of flow of intake air.

As a specific method for offsetting the stem of the specific intakevalve so as to improve the manner in which the intake air flows into thecylinder, the stem may be offset in a direction perpendicular to thedirection of flow of intake air as viewed in a horizontal projectionplane, and may be further offset to the downstream side with respect tothe center of the specific intake valve in the direction of flow ofintake air, or may be further offset to the upstream side with respectto the center of the specific intake valve in the direction of flow ofintake air. Namely, the stem of the specific intake valve may be offsetaway from the plane that contains the center axis of the cylinder and isparallel to the flow of intake air. If the stem of the specific intakevalve is further offset to the downstream side with respect to thecenter of the specific intake valve in the direction of flow of intakeair, the flow of intake air may not be smoothly formed right above anddownstream of the umbrella portion of the specific intake valve. It is,therefore, preferable that the stem is offset in the manner as describedabove, namely, is offset to the upstream side with respect to the centerof the specific valve, in the direction of flow of intake air.

The specific intake valve may be formed such that a portion of theumbrella portion of the specific intake valve, which corresponds to theinner passage region, has a smaller volume than a portion of theumbrella portion which corresponds to the outer passage region.

In the engine as described just above, the inner passage region ofintake air located close to the center of the combustion chamber can bemade larger than the outer passage region, and therefore, an increasedamount of intake air can be caused to flow toward the middle of thecombustion chamber when the valve lift of the intake valve is in a smallto middle range. Thus, the intake air can be introduced into thecylinder in a favorable manner so as to produce a whirling airflow inthe cylinder, from the time when the valve lift of the intake valve isin a small to middle range. Also, even where the degree of offsetting ofthe stem, i.e., the offset amount of the stem, is reduced to be smallerthan that of the intake valve in which the stem is offset without makingthe volume of the inner passage region larger, an equivalent effect canbe provided, and therefore, the strength of the intake valve can befavorably maintained.

It is preferable that the umbrella portion of the specific intake valveis smoothly formed over the entire circumference thereof so as not toimpede flow of intake air. To smoothly form the umbrella portion of thespecific intake valve, at least a part of the umbrella portion of thespecific intake valve may be formed in the shape of an arc in crosssection. In this regard, a portion of the umbrella portion of thespecific intake valve corresponding to the inner passage region and aportion corresponding to the outer passage region may be both formed inthe shape of arcs in cross section, and the radius of curvature of theportion corresponding to the inner passage region may be made smallerthan that of the portion corresponding to the outer passage region, sothat the volume of the portion corresponding to the inner passage regioncan be easily made smaller than the portion corresponding to the outerpassage region, as described above.

The specific intake valve may further include a rotation preventingdevice that prevents the specific intake valve from rotating about thecenter axis of the stem of the specific intake valve.

When the specific intake valve rotates about the stem, the intake portmay not be properly closed. This problem can be eliminated by providingthe specific intake valve with the above-mentioned rotation preventingdevice.

In the engine as described above, the intake air can be introduced intothe cylinder in a favorable manner so as to produce a whirling airflowin the cylinder, from the time when the valve lift of the intake valveis in a small to middle range.

In another preferred embodiment of the invention, the intake valve is aspecific intake valve in which the stem is inclined, when the intakevalve is in a closed state, such that a distal end of the stem islocated closer to a plane that contains a center axis of a cylinder andis substantially orthogonal to the axis of the crankshaft, than a centerof a bottom face of the umbrella portion, in a direction substantiallyparallel to the axis of the crankshaft.

In the engine in which the stem of the specific intake valve is inclinedin the manner as described above, an increased flow of intake air isdrawn toward the middle of the combustion chamber, so that themainstream of intake air that flows into the cylinder toward the middleof the combustion chamber can be increased or intensified. Thus,according to the preferred embodiment, the intensity of a whirlingairflow produced in the cylinder can be enhanced even when the valvelift of the intake valve is in a small to middle range, and the intakeair can be introduced into the cylinder in a favorable manner so as toproduce such a whirling airflow in the cylinder, from the time when thevalve lift of the intake valve is in a small to middle range.

In the above-described engine in which the stem of the specific intakevalve is inclined, the umbrella portion of the intake valve is alsoinclined so that the intake air that flows along a portion of theumbrella portion of the specific intake valve which is closer to thecenter of the combustion chamber than the stem is particularly directedso as to be dispersed toward the periphery of the combustion chamber.Generally, the mainstream of intake air is more likely to beconcentrated at around the middle of the combustion chamber as the liftof the intake valve becomes higher, resulting in an increased velocityof flow of intake air that flows toward the middle of the combustionchamber. In the engine according to the preferred embodiment asdescribed above, on the other hand, the intake air that flows into thecylinder when the lift of the intake valve is in a middle to high rangeis favorably prevented from hitting against the wall of the cylinder atan excessively high velocity, and the intensity of a whirling airflowproduced in the cylinder will not be reduced or less likely to bereduced. In the above description of the preferred embodiment, thephrase that “the specific intake valve is in a closed state” is used fordefining the specific intake valve as that being in a certain state(i.e., closed state) by way of example. The same phrase will be used inthe description of the following embodiment.

The stem of the specific intake valve may be inclined, when the specificintake valve is in a closed state, such that the distal end of the stemis located closer to an exhaust port than the center in a direction offlow of intake air.

With the above arrangement, the intake air that flows into the cylinderalong the umbrella portion can be further dispersed toward the bottomdead center of the cylinder. Thus, the intake air flowing into thecylinder is favorably prevented from hitting against the wall of thecylinder at an excessively high flow velocity, which would result in areduction of the intensity of a whirling airflow produced in thecylinder. Furthermore, in the engine as described above, it may bepossible to prevent the mainstream of intake air from being concentratedtoo much at around the middle of the combustion chamber, and furtherimprove the intensity of the whirling airflow, depending upon the degreeby which the stem of the specific intake valve is inclined in thedirection of flow of intake air.

Thus, in the engine as described above, the intake air can be introducedinto the cylinder in a favorable manner so as to produce a whirling flowin the cylinder from the time when the valve lift of the intake valve isin a small to middle range, and at the same time the intake air isprevented from hitting against the wall of the cylinder at anexcessively high velocity, which would result in a reduction of theintensity of the whirling airflow produced in the cylinder.

BRIEF DESCRIPTION OF THE DRAWINGS

The features, advantages, and technical and industrial significance ofthis invention will be better understood by reading the followingdetailed description of preferred embodiments of the invention, whenconsidered in connection with the accompanying drawings in which:

FIG. 1 is a view schematically showing an intake port 10A along with aprincipal part of an internal combustion engine 50A;

FIG. 2 is a schematic view of the intake ports 10A that is illustratedthree-dimensionally;

FIG. 3 is a view schematically showing the intake port 10A as viewed ina horizontal plane on which it is projected;

FIG. 4 is a graph showing the relationship between the valve lift andthe tumble intensity with respect to the intake port 10A and aconventional intake port 10X;

FIG. 5 is a view schematically showing an intake port 10B as viewed in ahorizontal plane on which it is projected;

FIG. 6 is a view schematically showing an intake port 10C as viewed in ahorizontal plane on which it is projected;

FIG. 7 is a view schematically showing a principal part of an internalcombustion engine 100A associated with one cylinder, as viewed in avertical cross-section;

FIG. 8 is a view schematically showing a principal part of the internalcombustion engine 100A associated with one cylinder, as viewed in ahorizontal plane on which that part of the engine 100A is projected;

FIG. 9 is a view schematically showing intake valves 155A in across-section taken along line A-A as shown in FIG. 8;

FIG. 10 is a graph showing the relationship between the tumble intensityand the valve lift;

FIG. 11 is a graph showing the relationship between the offset amount L,and the tumble intensity, valve strength and the flow rate of air;

FIG. 12 is a view schematically showing an intake valve 155Ab that isoriented in the same direction as in FIG. 8, as viewed in a directionperpendicular to a bottom face of its umbrella portion ub;

FIG. 13 is a view similar to that of FIG. 8 illustrating the principalpart of the engine 100A, in which the offset amount L is set to belarger than D/4;

FIG. 14 is a graph schematically showing the distribution of the flowvelocity measured at the middle of a combustion chamber 154;

FIG. 15 is a graph showing the relationship between the offset amount Land the tumble intensity, with respect to the case where a stem stm isoffset to the upstream side in a direction F of flow of intake air, andthe case where the stem stm is offset to the downstream side;

FIG. 16 is a view schematically showing an intake valve 155Bb in amanner similar to that of FIG. 12;

FIG. 17 is a graph schematically showing the distribution of the flowvelocity measured at the middle of the combustion chamber 154;

FIG. 18A, FIG. 18B and FIG. 18C are views showing the fuel consumptioncharacteristic of an internal combustion engine 100B during lean-burnoperation and the output performance thereof during high-load operation;

FIG. 19 is a view schematically showing intake valves 155C in across-section similar to the A-A cross-section shown in FIG. 8;

FIG. 20 is a view schematically showing the intake valve 155C in amanner similar to that of FIG. 12;

FIG. 21 is a graph schematically showing the distribution of the flowvelocity measured at the middle of the combustion chamber 154;

FIG. 22 is a view schematically showing a principal part of an internalcombustion engine 100D associated with one cylinder, as viewed in ahorizontal plane on which that part of the engine 100D is projected;

FIG. 23 is a view schematically showing a principal part of an internalcombustion engine 100E associated with one cylinder, as viewed in ahorizontal plane on which that part of the engine 100E is projected;

FIG. 24 is a graph showing the relationship between the tumble intensityand the valve lift;

FIG. 25 is a view schematically showing a principal part of an internalcombustion engine 200A associated with one cylinder, as viewed in avertical cross-section;

FIG. 26 is a view schematically showing a principal part of the internalcombustion engine 200A associated with one cylinder, as viewed in ahorizontal plane on which that part of the engine 200A is projected;

FIG. 27 is a view schematically showing intake valves 255A in across-section taken along line B-B as shown in FIG. 26;

FIG. 28A and FIG. 28B are views schematically showing the patterns offlow of intake air that flows into the cylinder;

FIG. 29 is a graph showing the relationship between the tumble intensityand the valve lift;

FIG. 30A and FIG. 30B are views schematically showing the patterns offlow of intake air that flows into the cylinder;

FIG. 31A and FIG. 31B are graphs schematically showing the distributionof the flow velocity measured at the middle of a combustion chamber 254;

FIG. 32 is a view schematically showing a principal part of an internalcombustion engine 200B associated with one cylinder, as viewed in ahorizontal plane on which that part of the engine 200B is projected, ina manner similar to that of FIG. 26;

FIG. 33 is a view schematically showing an intake valve 255Bb alone,which is oriented in the same direction as in FIG. 32;

FIG. 34A and FIG. 34B are graphs schematically showing the distributionof the flow velocity measured at the middle of the combustion chamber254;

FIG. 35A, FIG. 35B and FIG. 35C are views showing the fuel consumptioncharacteristic of the internal combustion engine 200B during lean-burnoperation and the output performance thereof during high-load operation;

FIG. 36 is a view schematically showing a principal part of an internalcombustion engine 200C associated with one cylinder, as viewed in ahorizontal plane on which that part of the engine 200C is projected;

FIG. 37 is a view schematically showing a principal part of an internalcombustion engine 200D associated with one cylinder, as viewed in ahorizontal plane on which that part of the engine 200D is projected;

FIG. 38 is a graph showing the relationship between the tumble intensityand the valve lift; and

FIG. 39A and FIG. 39B are views schematically showing conventionalintake ports 10X and 10Y, along with a combustion chamber 54, intakevalves 55 and exhaust valves 56.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following description and the accompanying drawings, the presentinvention will be described in more detail with reference to exemplaryembodiments.

Initially, a first embodiment of the invention will be described. FIG. 1schematically shows an intake port 10A (that represents intake port 10Aaand intake port 10Ab) having an intake port design of an internalcombustion engine according to the first embodiment of the invention,along with a principal part of the internal combustion engine 50A. Theengine 50A is a direct fuel injection type gasoline engine in which thefuel is injected directly into cylinders. It is, however, to beunderstood that the invention is not limitedly applied to this type ofengine, but may be applied to other types of engines, rather than thedirect fuel injection type gasoline engine. While the engine 50A is anin-line four-cylinder engine having four cylinders arranged in a line,the cylinder arrangement and the number of cylinders are not limited tothose of the engine 50A, but may be selected as appropriate. While aprincipal part of the engine 50A, more specifically, a principal part ofa cylinder 51 a as a typical cylinder, is illustrated in thisembodiment, the rest of the cylinders are constructed similarly.

The engine 50A includes a cylinder block 51, a cylinder head 52A, apiston 53, and other components. The cylinder 51 a having a generallycylindrical shape is formed in the cylinder block 51. The piston 53 isreceived in the cylinder 51 a. A cavity 53 a that serves to guide tumbleflow T is formed in the top face of the piston 53. The cylinder head 52Ais fixed to the upper face of the cylinder block 51. A combustionchamber 54 is formed as a space surrounded by the cylinder block 51,cylinder head 52A and the piston 63. The cylinder head 52A is formedwith intake ports 10A (10Aa and 10Ab) through which intake air is drawnto the combustion chamber 54, and exhaust ports 20 (20 a and 20 b)through which combustion gas is discharged from the combustion chamber54. Also, an intake valve 55 for opening and closing a channel of eachintake port 10A and an exhaust valve 56 for opening and closing achannel of each exhaust port 20 are mounted in the cylinder head 52A. Inaddition, an ignition plug and a fuel injection valve (not shown), forexample, are mounted in the cylinder head 52A.

The intake air flows into the combustion chamber 54 through the intakeport 10A after being deflected by an airflow control valve (not shown),so as to create a high-intensity tumble flow T in the combustion chamber54. While an inlet-side opening of the intake port 10A is formed in aside face of the cylinder head 52A in this embodiment, the intake port10A may be an upright port whose inlet-side opening is formed in theupper face of the cylinder head 52A. Also, a whirling airflow producedin the combustion chamber 54 is not limited to the tumble flow T, butmay be, for example, a reverse tumble flow that circulates in thedirection opposite to that of the tumble flow T as shown in FIG. 1, or aslanting tumble flow as a combination of the tumble flow T and a swirlflow.

FIG. 2 is a schematic view that illustrates the intake ports 10Athree-dimensionally. The vertical direction as seen in FIG. 2 denotes adirection parallel to a direction in which the cylinder 51 a extends,and the horizontal direction denotes a direction orthogonal to thatdirection. When viewed from the upper face of the cylinder head 52A, theintake ports 10A (10Aa, 10Ab) are projected onto a horizontal plane S asshown in FIG. 2. Vertical planes G orthogonal to the axis of thecrankshaft divide the horizontal plane S into the inner side and theouter side. In FIG. 2, a vertical plane G1 containing a starting pointP1 and a vertical plane G2 containing an end point P2 are respectivelyillustrated. In this connection, the starting point P1 is a point ofintersection of the streamline of the intake port 10A and the inlet-sideopening plane of the intake port 10A as projected on the horizontalplane S, and the end point P2 is a point of intersection of thestreamline of the intake port 10A and the center axis of the intakevalve 55 as projected on the horizontal plane S. In this embodiment, thecenter axis of the intake valve 55 is contained in the vertical planeG2. It is understood from FIG. 2 that the starting point P1 is locatedon the inner side of the vertical plane G2. It is also understood that aprojected streamline F of the intake port 10A, which is the streamlineprojected on the horizontal plane S, is curved inwardly of the verticalplane G1. The horizontal plane S and the vertical plane G1 cross eachother to form a straight line L1 (see FIG. 3), and the horizontal planeS and the vertical plane G2 cross each other to form a straight line L2(see FIG. 3). In other words, the starting point P1 is located closer tothe center of the combustion chamber than the straight line L2 thatcontains the end point P2 and extends in the direction orthogonal to theaxis of the crankshaft on the horizontal plane, and the streamline ofthe intake port as projected on the horizontal plane is curved to becloser to the center of the combustion chamber than the straight line L1that contains the starting point P1 and extends in the directionorthogonal to the axis of the crankshaft on the horizontal plane.

FIG. 3 schematically shows the intake ports 10A (10Aa, 10Ab) asprojected on the horizontal plane. In FIG. 3, the combustion chamber 54and the intake and exhaust valves 55, 56 are illustrated along with theintake ports 10A. As shown in FIG. 3, the starting point P1 is locatedon the inner side of the straight line L2. The projected streamline F ofthe intake port 10A is curved inwardly of the straight line L1. Withthis arrangement, a mainstream of intake air, which has been deflectedby the airflow control valve, is directed mainly in the first half ofthe curved portion so as to be introduced into the combustion chamber 54from between a valve stem portion of the intake valve 55 and the innerwall of the intake port 10A. Namely, even if the positions of thestarting point P1 and end point P2 have the relationship as shown inFIG. 3, the intake port 10A having the thus curved intake port design isable to direct the mainstream in the manner as described above. Thecurved portion may be designed, more specifically, the position, lengthand degree of curvature of the curved portion may be determined,depending upon the specifications, such as an angle of inclination ofthe intake port 10A.

FIG. 4 shows the relationship between the valve lift and the tumbleintensity, with respect to the intake port 10A having the intake portdesign according to the present embodiment and the conventional intakeport 10X as a comparative example. The intake port 10X is designed asshown in FIG. 39A. In the present embodiment, the mainstream of intakeair is focused on the inner side of the intake port 10A, so that a flowthat conforms to the umbrella shape of the intake valve 55 is formedwhen the valve lift is in a small to middle range, and the intake air ismost smoothly introduced into the combustion chamber 54. As a result,the tumble intensity can be enhanced particularly in the small to middlerange of the valve lift, as compared with the intake port 10X. As isunderstood from the above description, the intake port 10A provides anintake port design of the engine which makes it possible to intensifythe whirling airflow produced in the combustion chamber, even when thevalve lift of the intake valve 55 is in a small to middle range.

Next, an intake port 10B having an intake port design according to asecond embodiment of the invention will be described. The intake port10B is different from the intake port 10A having the intake port designaccording to the first embodiment, in that the starting point P1 islocated to be contained on a straight line L3 that contains the endpoint P2 and extends in a direction orthogonal to the axis of thecrankshaft on the horizontal plane S, and that the projected streamlineF of the intake port 10B as projected on the horizontal plane S iscurved inwardly of the straight line L3. FIG. 5 schematically shows theintake ports 10B (10Ba, 10Bb) when projected on the horizontal plane.FIG. 5 also shows the combustion chamber 54 and the intake and exhaustvalves 55, 56 along with the intake ports 10B. It is understood fromFIG. 5 that the starting point P1 is located on the straight line L3 inthe intake port 10B, and that the projected streamline F of the intakeport 10B is curved inwardly of the straight line L3.

With the above arrangement, the mainstream of the intake air is directedmainly before passing the curved portion so as to be introduced into thecombustion chamber 54 from between the valve stem portion of the intakevalve 55 and the inner wall of the intake port 10B. Namely, even if thepositions of the starting point P1 and end point P2 have therelationship as shown in FIG. 5, the intake port 10B having the thuscurved intake port design is able to direct the mainstream in the manneras described above. As is understood from the above description, theintake port 10B provides an intake port design of the engine which makesit possible to intensify the whirling airflow produced in the combustionchamber, even when the valve lift of the intake valve 55 is in a smallto middle range.

Next, an intake port 10C having an intake port design according to athird embodiment of the invention will be described. The intake port 10Cis different from the intake port 10A having the intake port designaccording to the first embodiment, in that the starting point P1 is notlocated on the inner side of the straight line L2 (namely, is located onthe outer side), and the projected streamline F of the intake port 10Cas projected on the horizontal plane S is curved inwardly of thestraight line L2. FIG. 6 schematically shows the intake ports 10C (10Ca,10Cb) when projected on the horizontal plane. FIG. 6 also shows thecombustion chamber 54 and the intake and exhaust valves 55, 56 alongwith the intake ports 10C. As is understood from FIG. 6, the startingpoint P1 is located on the outer side of the straight line L2, and theprojected streamline F of the intake port 10C is curved inwardly of thestraight line L2.

With the above arrangement, the mainstream of the intake air is directedmainly before passing the curved portion so as to be introduced into thecombustion chamber 54 from between the valve stem portion of the intakevalve 55 and the inner wall of the intake port 10C. Namely, even if thepositions of the starting point P1 and end point P2 have therelationship as shown in FIG. 6, the intake port 10C having the thuscurved intake port design is able to direct the mainstream in the manneras described above. When the starting point P1 is located on the outerside of the straight line L2, an intake port may be formed to provide aprojected streamline F that is substantially identical with the straightline connecting the starting line P1 with the end point P2, so that theintake air is directed inwards as a whole and is thus introduced intothe combustion chamber 54. However, the intake port 10C having theintake port design according to the present embodiment can enhance thedirectional characteristics of the mainstream. Where the starting pointP1 is located relatively close to the straight line L2, for example, thecurved form of the intake port 10C becomes particularly effective. As isunderstood from the above description, the intake port 10C provides anintake port design of the engine which makes it possible to intensifythe whirling airflow produced in the combustion chamber, even when thevalve lift of the intake valve 55 is in a small to middle range.

The illustrated embodiments are preferred embodiments of the invention.It is, however, to be understood that the invention is not limited tothese embodiments, but may be otherwise embodied with variousmodifications without departing from the principle of the invention. Forexample, while the intake ports 10A, 10B and 10C are independent portsin the illustrated embodiments, the intake port is not limited to thistype, but may be a Siamese port in which the intake passage is dividedinto two branch passages at the downstream side, which join into asingle passage at the upstream side.

Next, a fourth embodiment of the invention will be described.

FIG. 7 schematically shows a principal part of an internal combustionengine 100A according to the fourth embodiment of the invention. Moreparticularly, FIG. 7 shows one cylinder of the engine 100A as viewed invertical cross-section. The engine 100A is a direct fuel injection typegasoline engine, and employs a two-intake-valve structure in which eachcylinder is provided with two intake valves. It is, however, to beunderstood that the engine 100A is not limited to any particular typeprovided that the invention can be effectively practiced. For example,the engine may be a so-called lean-burn engine, or an engine having, forexample, a three-intake-valve structure as described later. The engine100A may also have an appropriate number of cylinders and an appropriatecylinder arrangement.

The internal combustion engine 100A has a cylinder block 151, a cylinderhead 152, a piston 153, and other components. A cylinder 151 a having agenerally cylindrical shape is formed in the cylinder block 151, and thepiston 153 is received in the cylinder 151 a. The cylinder head 152 isfixed to the cylinder block 151. A combustion chamber 154 is formed as aspace surrounded by the cylinder block 151, cylinder head 152 and thepiston 153. The cylinder head 152 is formed with intake ports 110Aa and110Ab (which will be simply and generically called “intake port 110A”,this way of calling being applied to other components) through which theintake air is introduced into the combustion chamber 154 (or into thecylinder), and exhaust ports 120 (120 a and 120 b) through whichcombustion gas is discharged from the combustion chamber 154.Furthermore, an intake valve 155A for opening and closing each intakeport 110A and an exhaust valve 156 for opening and closing each exhaustport 120 are respectively mounted in the cylinder head 152. The engine100A is provided with a rotation preventing means (not shown). Therotation preventing means may be implemented by, for example, forming aslit that extends in a direction in which a stem stm of the intake valve155A extends, in the stem stm, and providing the cylinder head 152 witha stem holding part that engages with the slit. In this embodiment, theintake valves 155A (155Aa, 155Ab) are regarded as specific intakevalves.

An ignition plug 157 is mounted in the cylinder head 152 such that itselectrode protrudes from above into the combustion chamber 154. A fuelinjection valve (not shown) is mounted in the cylinder head 152 suchthat its injection hole protrudes into the intake port 110A. The fuelinjection valve is adapted to inject fuel directly into the cylinder 151a on the intake stroke. The fuel injection valve is not limited to thistype or position, but may be mounted in the cylinder head 152 at aposition closer to the cylinder block 151 than the intake port 110A suchthat its injection hole protrudes into the combustion chamber 154, or ata position above the combustion chamber 154. The intake air flowing fromthe intake port 110A into the cylinder 151 a creates a whirling airflowin the cylinder. In this embodiment, the whirling airflow is,specifically, in the form of a tumble flow T as shown in FIG. 7. Acavity that serves to guide the tumble flow T may be formed in the topface of the piston 153.

FIG. 8 schematically shows a principal part of the engine 100Aassociated with one cylinder, as viewed in a horizontal plane on whichthat part of the engine 100A is projected. FIG. 9 schematically showsthe intake valves 155A (155Aa, 155Ab) in a cross-section taken alongline A-A in FIG. 8. As shown in FIG. 8, the intake port 110A extendslong so as to cause intake air to flow toward the middle of thecombustion chamber 154 as viewed in the horizontal projection plane.With this arrangement, the intake air flows through the intake port 110Atoward the middle of the combustion chamber 154, while forming amainstream of intake air that produces a tumble flow T in the cylinder.Thus, the direction F of flow of intake air is substantially orthogonalto an axis L4 substantially parallel to the axis of the crankshaft, asshown in FIG. 8. In this connection, the direction F of flow of intakeair indicates the direction in which the mainstream of the intake airflows so as to produce a tumble flow T in the cylinder, and may berepresented by a direction of extension of the intake port 110A in aportion where the stem stm is placed, depending upon the shape or designof the intake port 110A.

As shown in FIG. 8 and FIG. 9, the stem stm is offset such that thecenter axis C1 of the stem stm does not contain the center P2 of thebottom face of an umbrella portion ub of the intake valve 155A. In thepresent embodiment, the stem stm is offset in a direction substantiallyperpendicular to the direction F of flow of intake air as viewed in thehorizontal projection plane, in other words, in a directionsubstantially parallel to the axis L4, as shown in FIG. 8. Here, anintake air channel of the intake port 110A through which intake airpasses is divided by a plane S1 containing the center axis C1 of thestem stm is divided into two regions, namely, an inner passage regioninr and an outer passage region otr. The stem stm is offset such thatthe inner passage region inr located closer to the center of thecombustion chamber 154 becomes larger than that of the case where thestem stm is not offset, more specifically, such that the inner passageregion inr becomes larger than the outer passage region otr, as shown inFIG. 8. Thus, the intake air caused to flow toward the middle of thecombustion chamber 154 is more likely to be prevented from interferingwith the stem stm, as shown in FIG. 8. Accordingly, flow of intake airdirected toward the middle of the combustion chamber 154 is more likelyto be formed when the valve lift of the intake valve is in a small tomiddle range.

In FIG. 8, the plane S1 is further assumed to be a plane substantiallyparallel to the center axis C3 of the cylinder, and therefore, the planeS1 is represented by a straight line in FIG. 8. When a cross-section (asuitable section indicating the shape of the intake port 110A at acertain position in the direction of extension of the intake port 110A,for example, a section perpendicular to the intake port 110A or asection perpendicular to the direction F of flow of intake air) of theintake port 110A is taken at a position where the stem stm, which may bereceived in a stem guide (not shown), is disposed in the intake port110A, the cross-section of the intake port 110A is divided into twoportions by the plane S1, and these two portions respectively correspondto parts of the inner and outer passage regions inr, otr.

In this respect, it is preferable that a portion corresponding to theinner passage region inr is larger than a portion corresponding to theouter passage region otr in all of the sections of the intake port 110A,but this is not always true, depending upon the design of the intakeport 110A. As described above, the inner and outer passage regions inr,otr are intake-air passage regions taken at the position where the stemstm is disposed in the intake port 110A. Thus, the inner and outerpassage regions inr, otr do not include intake-air passage regionspartitioned by the plane S1 at the location where the stem stm is notdisposed in the intake port 110A.

FIG. 10 shows the relationship between the tumble intensity and thevalve lift. The tumble intensity is represented by the number of tumblerevolutions. FIG. 10 shows the results of comparison in terms of thetumble intensity between the engine 100A and an internal combustionengine 100X having intake valves whose stems stm are not offset, inplace of the intake valves 155A. The engine 100X is substantiallyidentical with the engine 100A except that the engine 100X has differentintake valves from those of the engine 100A. It is understood from FIG.10 that the tumble intensity is improved from the time when the valvelift of the intake valve is in a small to middle range in the engine100A, as compared with the engine 100X.

Next, the offset amount L of the stem stm will be described in detail.FIG. 11 shows the relationship between the offset amount L, and thetumble intensity, valve strength and the flow rate of air. FIG. 12schematically shows the intake valve 155Ab disposed on the right-handside in FIG. 8, as viewed in a direction perpendicular to the bottomface of the umbrella portion ub, when the intake valve 115Ab is orientedin the same direction as that in FIG. 8. As shown in FIG. 12, the offsetamount L is established which indicates a distance from the center P2 tothe stem stm (more specifically, point P5 as a point of intersection ofthe bottom face of the umbrella portion ub and the center axis C1 of thestem stm). In FIG. 12, D represents a valve outside diameter, which isthe outside diameter of the umbrella portion ub of the intake valve155Ab. Also, the arrow “POSITIVE” associated with the offset amount Lindicates a direction in which the stem stm is displaced away from thecenter axis C3 of the cylinder in a direction substantially orthogonalto the direction F of flow of intake air.

As shown in FIG. 11, when the offset amount L is in the range of 0(zero) to D/12, the tumble intensity increases as the offset amount Lincreases. Even where the offset amount L further increases to be largerthan D/12, the tumble intensity increases at a low rate as the offsetamount L increases. Thus, if the offset amount L is in the range asindicated by the following expression (1), the tumble intensity can beimproved.

0<L  (1)

If the offset amount L further increases to be around D/4, on the otherhand, the valve strength of the intake valve 155A starts being largelyreduced. If the offset amount L further increases to be larger than D/4,the valve strength is largely reduced, and the flow rate of air startsbeing largely reduced. The reduction in the flow rate of air isconsidered as being caused by an extreme reduction in the amount ofintake air passing the outer passage region otr. It is thus preferablethat the offset amount L is within a permissible range as indicated bythe following expression (2).

0 <L≦D/4  (2)

Since the tumble intensity is in the middle of largely increasing whenthe offset amount L is in the range of 0 to D/12, as shown in FIG. 11,it is further preferable that the offset amount L is in a recommendedrange as indicated by the following expression (3).

D/12≦L≦D/4  (3)

FIG. 14 schematically shows the distribution of the velocity of flow ofair measured at the middle of the combustion chamber 154. FIG. 14 showsthe results of comparison in terms of the flow velocity between theengine 100A and the engine 100X. When the offset amount L is within therange as indicated by the following expression (4), the flow velocityobtained in the engine 100A is larger than obtained in the engine 100X,but is less than a predetermined value α.

0<L<D/12  (4)

If the offset amount L is within the recommended range as indicated bythe above expression (3), on the other hand, the flow velocity becomesequal to or larger than the predetermined value α. As is understood fromthis result, it is further preferable that the offset amount L is withinthe recommended range as indicated by the expression (3). The range asindicated by the above expression (3) or (4) is considered as a rangethat can provide a reasonable effect even in the case where the positionof the stem stm as viewed in the direction F of flow of intake air ischanged. In FIG. 12, area AR1 represents an area that corresponds to therange as indicated by the expression (4) and is considered as providinga reasonable effect, and area AR2 represents an area that corresponds tothe range as indicated by the expression (3) and is considered asproviding a reasonable effect. In the engine 100A constructed asdescribed above, the intake air can be introduced into the cylinder in afavorable manner so as to produce tumble flow T in the cylinder, fromthe time when the valve lift of the intake valve is in a small to middlerange.

Next, a fifth embodiment of the invention will be described. An internalcombustion engine 100B according to the fifth embodiment is basicallyidentical with the engine 100A of the fourth embodiment, except that anintake valve 155B (which represents intake valve 155Ba and intake valve155Bb) of the engine 100B replaces the intake valve 155A of the engine100A. The intake valve 155B is different from the intake valve 155A inthat a stem stm of the intake valve 155B is further offset to theupstream side of the center P2 as viewed in the direction F of flow ofintake air. In this embodiment, the intake valve 155B is considered as aspecific intake valve.

FIG. 15 shows the relationship between the offset amount L and thetumble intensity with respect to the case where the stem stm is offsetto the upstream side in the direction F of flow of intake air and thecase where the stem stm is offset to the downstream side in the samedirection. FIG. 16, which is similar to FIG. 12, schematically shows theintake valve 155Bb disposed on the right-hand side, like the intakevalve 155Ab disposed on the right-hand side in FIG. 8. As shown in FIG.16, an installation angle θ is established which represents an acuteangle formed by a straight line L6 that is substantially orthogonal tothe direction F of flow of intake air and contains the center P2 and astraight line L7 that contains the center P2 and a point P5 that lies onthe center axis C1 of the stem stm. The arrow “POSITIVE” associated withthe installation angle θ in FIG. 16 indicates that the installationangle θ assumes a positive value when the stem stm is offset to theupstream side in the direction F of flow of intake air, as shown in FIG.16. In FIG. 16, the offset amount L is set in the same manner as in FIG.12.

As described above with regard to the fourth embodiment, it ispreferable that the offset amount L is within the recommended range asindicated by the above expression (3). If the installation angle θ isfurther set to be larger than about 70 degrees or smaller than about −70degrees, the stem stm will be contained in the area AR1. Thus, it ispreferable that the installation angle θ is within a permissible rangeas indicated by the following expression (5).

−70°≦θ≦70°  (5)

-   It is, however, to be noted that the tumble intensity is more or    less improved if the stem stm is located in the area AR1, and    therefore, a reasonable effect can be provided if the installation    angle θ is equal to or larger than −90° and is equal to or smaller    than 90°.

If the stem stm is offset to the upstream side with the installationangle θ set to 90°, the tumble intensity increases at a low rate as theoffset amount L increases, and then largely increases, as shown in FIG.16. If the stem stm is offset to the downstream side with theinstallation angle θ set to −90°, the tumble intensity decreases at alow rate as the offset amount L increases, and then largely decreases.This may be because when the stem stm is offset to the downstream side,it is difficult to smoothly form flow of intake air to the downstreamside, right above the umbrella portion ub. Thus, it is furtherpreferable that the installation angle 74 is in a recommended range asindicated by the following expression (6).

0°≦θ≦70°  (6)

Even in the case where the installation angle θ is set to within theabove-described range, the valve strength is reduced as shown in FIG. 15if the offset amount L is larger than D/4. Where the installation angleθ is established, therefore, it is preferable to form the stem stm sothat the offset amount L and the installation angle θ are in the rangesof the expression (3) and expression (6), respectively. In FIG. 16, areaAR3 represents an area corresponding to the case where the offset amountL is set to within the range as indicated by the expression (3), and theinstallation angle θ is set to within the range as indicated by theexpression (6). In the present embodiment, therefore, the stem stm isoffset so as to be contained in the area AR3 as shown in FIG. 16.

FIG. 17 schematically shows the distribution of the velocity of flow ofair measured at the middle of the combustion chamber 154. FIG. 17 showsthe results of comparison in terms of the flow velocity between theengine 100B and the engine 100X. When the stem stm of the intake valve155B is located in the area AR3, the engine 100B provides a flowvelocity that is larger than a predetermined value β. The predeterminedvalue β larger than the above-mentioned predetermined value α. Itfollows that the engine 100B provides a higher flow velocity than thatof the engine 100A as described above in the fourth embodiment of theinvention.

FIG. 18A, FIG. 18B and FIG. 18C show the fuel consumption characteristicof the engine 100B during lean-burn operation, and the outputperformance during high-load operation. The fuel consumptioncharacteristic during lean-burn operation is specifically indicated inFIG. 18A by the relationship between the fuel consumption rate of theengine 100B and the air-fuel ratio, and the output performance duringhigh-load operation is specifically indicated in FIG. 18B by therelationship between the engine torque and the engine speed of theengine 100B. In FIG. 18B, the output performance is indicated by thefull-load performance. FIG. 18A and FIG. 18B show the results ofcomparison between the engine 100X and the engine 100B. FIG. 18Cquantitatively indicates the fuel consumption reduction rate and outputperformance improvement rate that are expected to be achieved when theoffset amount L is within the ranges of the expression (2) andexpression (3), and also indicates the fuel consumption reduction rateand output performance improvement rate that are expected to be achievedwhen the installation angle θ is within the ranges of the expression (5)and expression (6).

When the stem stm is offset by an appropriate degree, a high-intensitytumble flow T can be formed in the cylinder and strong turbulence can beproduced from the time when the valve lift of the intake valve is in asmall to middle range. Therefore, the combustion characteristics areimproved during lean-burn operation, resulting in a reduction of thefuel consumption rate, and the output performance can be improved duringhigh-load operation. As shown in FIG. 18A, the fuel consumption rate isreduced in the engine 100B, as compared with the engine 100X, and alean-burn region in which the engine is operable at a lean air-fuelratio is expanded. Also, as shown in FIG. 18B, the engine torque isimproved over the entire range of the engine speed in the engine 100B ascompared with the engine 100X. It will be understood from FIG. 18B thatthe degree of improvement of the engine torque increases as the enginespeed decreases, and that the effect provided by offsetting the stem stmis greater as the engine speed is lower.

If the offset amount L is set to within the ranges of the expression (2)and the expression (3), the fuel consumption reduction rate and outputperformance improvement rate as quantitatively indicated in FIG. 18C areexpected to be achieved. If the installation angle θ is set to withinthe ranges of the expression (5) and the expression (6), the fuelconsumption reduction rate and output performance improvement rate asquantitatively indicated in FIG. 18C are expected to be achieved. In theengine 100B constructed as described above, the intake air can beintroduced into the cylinder in a favorable manner so as to producetumble flow T in the cylinder, from the time when the valve lift of theintake valve is in a small to middle range.

Next, a sixth embodiment of the invention will be described. An internalcombustion engine 100C according to the sixth embodiment issubstantially identical with the engine 100B according to the fifthembodiment, except that an intake valve 155C (which represents intakevalve 155Ca and intake valve 155Cb) of the engine 100C replaces theintake valve 155B of the engine 100B. The intake valve 155C is differentfrom the intake valve 155B in that the volume of a portion of theumbrella portion ub corresponding to the inner passage region inr ismade smaller than that of a portion of the umbrella portion ubcorresponding to the outer passage region otr. More specifically, whenthe portion corresponding to the outer passage region otr is rotatedabout the center axis C1 of the stem stm, to be superimposed on theportion corresponding to the inner passage region inr, these portions donot coincide with each other, and the portion corresponding to the innerpassage region inr is at least partially contained in the portioncorresponding to the outer passage region otr. In this embodiment, theintake valve 155C is considered as a specific intake valve.

FIG. 19 schematically shows the intake valves 155C (155Ca and 155Cb) incross section similar to the section A-A of FIG. 8. The intake valve155C is specifically formed such that the portion of the umbrellaportion ub corresponding to the inner passage region inr and the portioncorresponding to the outer passage region otr are both formed in theshape of an arc as viewed in a cross-section provided by a planecontaining the center axis C1, and such that the portion correspondingto the inner passage region inr has a smaller radius of curvature thanthe portion corresponding to the outer passage region otr. The portioncorresponding to the inner passage region inr, which has a radius R2 ofcurvature, and the portion corresponding to the outer passage regionotr, which has a radius R1 of curvature, are respectively formed so asto be smoothly connected with the stem stm, and the radius R2 ofcurvature is set to be smaller than the radius R1 of curvature.

In the sixth embodiment in which the umbrella portion ub is formed inthe manner as described above, the portion of the umbrella portion ubcorresponding to the inner passage region inr has a smaller volume thanthe portion corresponding to the outer passage region otr. Thus, theinner passage region inr can be made larger, and therefore, furtherincreased flow of intake air toward the middle of the combustion chamber154 can be formed when the valve lift of the intake valve is in a smallto middle range. The umbrella portion ub of the intake valve 155C issmoothly formed over the entire circumference so as not to impede flowof intake air. In this respect, the umbrella portions ub of the intakevalves 155A and 115B are formed in a similar manner.

FIG. 20 schematically shows the intake valve 155C (156Cb) disposed onthe right-hand side, like the intake valve 155A disposed on theright-hand side in FIG. 8, in a manner similar to FIG. 12 or FIG. 16. InFIG. 20, the offset amount L is set in the same manner as in FIG. 12,and the installation angle θ is set in the same manner as in FIG. 16. Inthe engine 100C having the intake valve 155C, further increased flow ofintake air toward the middle of the combustion chamber 154 can be formedas described above. Therefore, even if the offset amount L is madesmaller in the engine 100C than in the engine 100A or 100B, the engine100C can provide an effect equivalent to that provided by the engine100A or 100B. Thus, in the case of the engine 100C, the recommendedrange of the offset amount L can be expanded as indicated by thefollowing expression (7), as compared with the recommended range asindicated by the above expression (3).

D/24≦L≦D/4  (7)

-   In FIG. 20, area AR4 represents an area corresponding to the    recommended range as indicated by the above expression (7), in which    a reasonable effect is supposed to be provided.

In the case where the installation angle θ is established, if theinstallation angle θ is larger than about 80 degrees or smaller thanabout −80 degrees when the offset amount L is D/4, the stem stm iscontained in the above-mentioned area AR4. Thus, in the case of theengine 100C, the permissible range of the installation angle θ can beexpanded as indicated by the following expression (8), as compared withthe permissible range as indicated by the above expression (5).

−80°≦θ≦80°  (8)

As in the case of the engine 100B, it is preferable in the engine 100Cthat the stem stm is offset to the upstream side relative to the centerP2, in the direction F of flow of intake air. Thus, in the case of theengine 100C, the recommended range of the installation angle θ can beexpanded as indicated by the following expression (9), as compared withthe recommended range as indicated by the above expression (6).

0°≦θ≦80°  (9)

As the ranges of the offset amount L and the installation angle θ areexpanded as described above, the strength of the intake valve 155C canbe more favorably maintained. Also, in the case of the engine 100C, thestem stm is preferably formed so as to satisfy both of the rangesindicated by the above expressions (7) and (9). In FIG. 20, area AR5represents an area corresponding to the case where the offset amount Lis set to within the range as indicated by the expression (7) and theinstallation angle θ is set to within the expression (9).

FIG. 21 schematically shows the distribution of the velocity of flow ofair measured at the middle of the combustion chamber 154. FIG. 21 showsthe results of comparison in terms of the flow velocity among the engine100C, the engine 100B and the engine 100X. In the engine 100B and theengine 100C compared with each other in FIG. 21, the offset amount L isset to the same value within the recommended range indicated by theabove expression (3), and the installation angle θ is set to the samevalue within the recommended range indicated by the above expression(6). Also, in FIG. 21, the umbrella portion ub of the intake valve 155Bof the engine 100B is formed such that the portion of the umbrellaportion ub corresponding to the inner passage region inr and the portioncorresponding to the outer passage region otr have the same radius ofcurvature, and are smoothly connected with the stem stm. As shown inFIG. 21, the engine 100C provides a larger flow velocity than the engine100C. In the engine 100C constructed as described above, the intake aircan be introduced into the cylinder in a favorable manner so as to forma tumble flow T in the cylinder, from the time when the valve lift ofthe intake valve is in a small to middle range.

Next, a seventh embodiment of the invention will be described. Aninternal combustion engine 100D according to the seventh embodiment isdifferent from the engines 100A, 100B and 100C of the fourth throughsixth embodiments as described above, in that the engine 100D has athree-intake-valve structure, namely, each cylinder is provided withthree intake valves. FIG. 22, which is similar to FIG. 8, schematicallyshows a principal part of the engine 100D associated with one cylinder,as viewed in a horizontal plane on which that part of the engine 100D isprojected. In the engine 100D, stems stm of intake valves 155D (155Da,155Db) located at the opposite ends are offset in a manner similar tothat of the intake valve 155A. Furthermore, the installation angles θ ofthe intake valves 155Da, 155Db may be set in a manner similar to that ofthe intake valve 155B, and each of the intake valves 155Da, 155Db may beformed such that a portion of the umbrella portion ub corresponding tothe inner passage region inr has a smaller volume than a portioncorresponding to the outer passage region otr, as in the intake valve155C.

In the engine 100D as shown in FIG. 22, the intake valves 155Da, 155Dblocated at the opposite ends with respect to one cylinder are regardedas specific intake valves. With the specific intake valves thusprovided, increased flow of intake air toward the middle of thecombustion chamber 154 can be formed when the valve lift of the intakevalves is in a small to middle range, even where the engine 100D has thethree-intake-valve structure. In the engine 100D constructed asdescribed above, the intake air can be introduced into the cylinder in afavorable manner so as to produce a tumble flow T in the cylinder, fromthe time when the valve lift of the intake valve is in a small to middlerange.

Next, an eighth embodiment of the invention will be described. Aninternal combustion engine 100E according to the eighth embodiment issubstantially identical with the engine 100A according to the fourthembodiment, except that intake ports 110E (which represent intake port110Ea and intake port 110Eb) are further provided with an airflowcontrol valve 160 that deflects intake air in the intake ports 110E soas to create a high-intensity tumble flow T in the cylinder. Namely, theengine 100E is equivalent to the engine 100A that is further equippedwith the airflow control valve 160. The engines 100B, 100C and 100Daccording to the fifth through seventh embodiments may also be providedwith airflow control valves that operate in substantially the samemanner and provide substantially the same effect as the airflow controlvalve 160.

FIG. 23, which is similar to FIG. 8, schematically shows a principalpart of the engine 100E associated with one cylinder, as viewed in ahorizontal plane on which that part of the engine 100E is projected. Oneof the opposite edges of the airflow control valve 160 is pivotallysupported by a valve stem 161, and the other edge of the airflow controlvalve 160 is formed with a recessed portion K for directing intake airtoward the middle of the combustion chamber 154 when the valve 160 isclosed. To cause the recessed portion K to direct the intake air towardthe middle of the combustion chamber 154, the other edge of the airflowcontrol valve 160 is recessed such that the depth of the recessincreases as it approaches a portion of the valve 160 corresponding tothe middle of the combustion chamber 154, and such that the recessedportion K is expanded at the center thereof. When the airflow controlvalve 160 is fully closed or half open, the intake air flowing throughthe intake port 110E passes the recessed portion K, thereby to bedirected toward the middle of the combustion chamber 154. By directingthe intake air toward the middle of the combustion chamber 154 beforethe intake air flows into the cylinder, the amount of intake air flowingtoward the middle of the combustion chamber 154 can be increased. As aresult, the amount of intake air that interferes with the stem stm ofthe intake valve 155E can be further reduced, so that the intake air canbe introduced into the cylinder in a favorable manner.

FIG. 24 shows the relationship between the tumble intensity and thevalve lift. FIG. 24 shows the results of comparison in terms of thetumble intensity, between the engine 100E and an internal combustionengine 100Y having intake valves whose stems are not offset, in place ofthe intake valves 155E. The engine 100Y is substantially identical withthe engine 100E, except that the intake valves of the engine 100Y aredifferent from those of the engine 100E. It is understood from FIG. 24that the tumble intensity is improved when the airflow control valve 160is fully closed, rather than when the valve 160 is fully open. It isalso understood from FIG. 24 that in the engine 100E, as compared withthe engine 100Y, the tumble intensity is improved when the airflowcontrol valve 160 is fully closed from the time when the valve lift ofthe intake valves is in a small to middle range. In the engine 100Econstructed as described above, the intake air can be introduced intothe cylinder in a favorable manner so as to produce a tumble flow T inthe cylinder, from the time when the valve lift of the intake valve isin a small to middle range.

Next, a ninth embodiment of the invention will be described. FIG. 25schematically shows a principal part of an internal combustion engine200A according to the ninth embodiment of the invention. Moreparticularly, FIG. 7 shows one cylinder of the engine 200A as viewed invertical cross section. The engine 200A is a direct fuel injection typegasoline engine, and employs a two-intake-valve structure (i.e., eachcylinder is provided with two intake valves). It is, however, to beunderstood that the engine 200A is not limited to any particular typeprovided that the invention can be effectively practiced. For example,the engine may be a so-called lean-burn engine, or an engine having, forexample, a three-intake-valve structure as described later. The engine200A may also have an appropriate number of cylinders and an appropriatecylinder arrangement.

The internal combustion engine 200A has a cylinder block 251, a cylinderhead 252, a piston 253, and other components. A cylinder 251 a having agenerally cylindrical shape is formed in the cylinder block 251, and thepiston 253 is received in the cylinder 251 a. The cylinder head 252 isfixed to the cylinder block 251. A combustion chamber 254 is formed as aspace surrounded by the cylinder block 251, cylinder head 252 and thepiston 253. The cylinder head 252 is formed with intake ports 210Aa and210Ab (which will be simply and generically called “intake port 210A”,this way of calling being applied to other components) through which theintake air is introduced into the combustion chamber 254 (or into thecylinder), and exhaust ports 220 (220 a and 220 b) through whichcombustion gas is discharged from the combustion chamber 254.Furthermore, an intake valve 255A for opening and closing each intakeport 210A and an exhaust valve 256 for opening and closing each exhaustport 220 are respectively mounted in the cylinder head 252. In thisembodiment the intake valves 255A (255Aa, 255Ab) are regarded asspecific intake valves.

An ignition plug 257 is mounted in the cylinder head 252 such that itselectrode protrudes from above into the combustion chamber 254. A fuelinjection valve (not shown) is mounted in the cylinder head 252 suchthat its injection hole protrudes into the intake port 210A. The fuelinjection valve is adapted to inject fuel directly into the cylinder 251a on the intake stroke. The fuel injection valve is not limited to thistype or position, but may be mounted in the cylinder head 252 at aposition closer to the cylinder block 251 than the intake port 210A suchthat its injection hole protrudes into the combustion chamber 254, or ata position above the combustion chamber 254. The intake air flowing fromthe intake port 210A into the cylinder 251 a creates a whirling airflowin the cylinder. In this embodiment, the whirling airflow is,specifically, in the form of a tumble flow T as shown in FIG. 25. Acavity that serves to guide the tumble flow T may be formed in the topface of the piston 253.

FIG. 26 schematically shows a principal part of the engine 200Aassociated with one cylinder, as viewed in a horizontal plane on whichthat part of the engine 200A is projected. FIG. 27 schematically showsthe intake valves 255A (255Aa, 255Ab) in a cross-section taken alongline B-B in FIG. 26. As shown in FIG. 26, the intake port 210A extendslong so as to cause intake air to flow toward the middle of thecombustion chamber 254 as viewed in the horizontal projection plane.With this arrangement, the intake air flows toward the middle of thecombustion chamber 254, while forming a mainstream of intake air thatproduces a tumble flow T in the cylinder. In this embodiment, themainstream of intake air is formed by flow of intake air that flows inthe direction F of flow of intake air as shown in FIG. 26. In thisconnection, the direction F of flow of intake air indicates thedirection in which the mainstream of intake air flows so as to produce awhirling airflow in the cylinder, and may be represented by thedirection of extension of the intake port 210A in a portion where thestem stm is placed, depending upon the shape or design of the intakeport 210A.

In FIG. 26, straight line L4 is substantially parallel to the axis ofthe crankshaft, and plane S2 contains the center axis C3 of thecylinder, and is substantially orthogonal to the straight line L4,namely, is substantially orthogonal to the axis of the crankshaft. Asshown in FIG. 26 and FIG. 27, the stem stm of the intake valve 255A isinclined so that, while the intake valve 255A is in the closed state,the distal end P11 of the stem stm is closer to the above-indicatedplane S2 than the center P2 of the bottom face of the umbrella portionub, as viewed in a direction substantially parallel to the axis of thecrankshaft. The distal end P11 of the stem stm and the center P2 of thebottom face of the umbrella portion ub are both located on the centeraxis C1 of the stem stm In the engine 200A in which the intake valves255A are inclined as described above, an increased amount of intake airis drawn to the side closer to the middle of the combustion chamber 254,so that the mainstream of intake air that flows into the cylinder towardthe middle of the combustion chamber 254 can be increased orintensified. Thus, the engine 200A is arranged to improve the intensityof the tumble flow T even when the valve lift of the intake valves is ina small to middle range.

In the engine 200A in which the stems stem of the intake valves 255A areinclined, the umbrella portions ub of the intake valves 255A are alsoinclined as shown in FIG. 27. In the engine 200A thus constructed, theintake air that flows along the umbrella portion ub on the side closerto the center of the combustion chamber 254 than the stem stm isdirected in such a manner as to spread toward the periphery of thecombustion chamber 254, as shown in FIG. 26. In the engine 200A,therefore the intake air that flows into the cylinder when the valvelift of the intake valves is in a middle to high range is favorablyprevented from hitting against the inner wall of the cylinder 251 a atan excessively high velocity, which would result in a reduction of theintensity of the tumble flow T produced. Inter-stem angle θ2 shown inFIG. 27 is an acute angle formed between the center axes C1 of the stemsstm of the intake valves 255A (255Aa and 255Ab). The degree ofinclination of the intake valves 255A can be changed through setting ofthis inter-stem angle θ2.

In the engine 200A in which the mainstream of intake air is drawn towardthe middle of the combustion chamber 254, the distance Lv between thevalve seats on which the intake valves 255Aa, 255Ab rest as shown inFIG. 27 can be made larger than that in the case where the angle θ2 isset to 0°. Namely, in the engine 200A, it is possible to draw themainstream of intake air toward the middle of the combustion chamber 254without reducing the distance Lv between the valve seats. Thus, in theengine 200A, as compared with the engine in which the inter-stem angleθ2 is equal to 0°, it is possible to draw the mainstream of intake airtoward the middle of the combustion chamber 254 while assuringsufficient strength of the combustion chamber 254.

FIG. 28A and FIG. 28B schematically show the patterns of flow of intakeair that flows into the cylinder. More specifically, FIG. 28Aschematically shows the pattern of flow of intake air in one cylinder ofan internal combustion engine 200X as viewed in vertical cross sectionas in FIG. 25, and FIG. 28B schematically shows the pattern of flow ofintake air in one cylinder of the engine 200A as viewed in verticalcross section as in FIG. 25. The engine 200X is substantially identicalwith the engine 200A except that the inter-stem angle θ2 is set to 0° inthe engine 200X. In the engine 200X as shown in FIG. 28A, the intake airis likely to hit against a wall of the cylinder 251 a at an excessivelyhigh velocity when the valve lift of the intake valves is in a middle tohigh range, resulting in noticeable occurrence of flow Fs along the wallof the cylinder 251 a, which causes a reduction in the strength of thetumble flow T produced in the cylinder. In the engine 200A as shown inFIG. 28B, on the other hand, the intake air flows into the cylinderwhile being dispersed by an appropriate degree when the valve lift ofthe intake valves is in a middle to high range. Therefore, theabove-mentioned flow Fs along the wall of the cylinder 251 a is lesslikely to occur, and the otherwise possible reduction in the strength ofthe tumble flow T produced can be lessened or prevented.

FIG. 29 shows the relationship between the tumble intensity and thevalve lift. The tumble intensity is represented by the number of tumblerevolutions. FIG. 29 shows the results of comparison in terms of thetumble intensity between the engine 200X and the engine 200A. It isunderstood from FIG. 29 that the tumble intensity is improved in theengine 200A from the time when the valve lift of the intake valves is ina small to middle range, as compared with that of the engine 200X.

Next, the inter-stem angle θ2 will be explained in detail. In a commoninternal combustion engine, the inter-stem angle θ2 is set to 0°. If theangle θ2 is within a range as indicated by the following expression(10), on the other hand, the tumble intensity can be improved.

0°<θ2  (10)

As the inter-stem angle θ2 increases, however, it becomes physicallydifficult to appropriately place cams (not shown) for opening andclosing the intake valves 255A. Accordingly, it is preferable in view ofthe placement of the cams that the angle θ2 is within a permissiblerange as indicated by the following expression (11).

0°<θ2≦10°  (11)

When the angle θ2 is set to be larger than 0°, the intake air flowinginto the cylinder is dispersed toward the periphery of the combustionchamber 254. If the dispersion occurs excessively, however, the tumbleintensity may be reduced, rather than improved. FIG. 30A and FIG. 30Bschematically show the patterns of flow of intake air that flows intothe cylinder in the engine 200A in which the inter-stem angle θ2 is setto be larger than 6° and equal to or smaller than 10°0. Morespecifically, FIG. 30A schematically shows the pattern of flow of intakeair in one cylinder of the engine 200A as viewed in vertical crosssection as in (the case of) FIG. 25, and FIG. 30B schematically showsthe form of flow (manner of flowing) of intake air in (one cylinder of)the engine 200A as viewed in a cross-section similar to (the) B-B(cross-)section as shown in FIG. 26.

If the inter-stem angle θ2 is set to be larger than 6° and equal to orsmaller than 10°, there may arise a situation where the intake airflowing into the cylinder is excessively dispersed, as shown in FIGS.30A and 30B, depending upon the shape of the combustion chamber 254. Ifthe angle θ2 is set to be larger than 0° and less than 1°, on the otherhand, a significant effect cannot be expected. Thus, it is furtherpreferable that the inter-stem angle θ2 is in a recommended range asindicated by the following expression (12).

1°≦θ2≦6°  (12)

FIG. 31A and FIG. 31B schematically show the distribution of thevelocity of flow of air measured at the middle of the combustion chamber254. FIG. 31A and FIG. 31B show the results of comparison in terms ofthe flow velocity between the engine 200A in which the inter-stem angleθ2 is within the range of the above expression (12), and the engine X.More specifically, FIG. 31A shows the distribution of the flow velocitywhen the valve lift of the intake valves is in a small to middle range,and FIG. 31B shows the distribution of the flow velocity when the valvelift of the intake valves is in a middle to high range. When theinter-stem angle θ2 is within the range as indicated by the expression(12), the engine 200A provides a flow velocity that is larger than apredetermined value α2 as shown in FIG. 31A when the valve lift of theintake valves is in a small to middle range. The predetermined value α2is larger than the maximum flow velocity that can be achieved by theengine 200X. When the valve lift of the intake valves is in a middle tohigh range, on the other hand, the engine 200A provides a flow velocitythat is equal to or larger than a predetermined value β2, as shown inFIG. 31B. The predetermined value β2 is larger than the predeterminedvalue α2, and is smaller than a predetermined value γ2. If the flowvelocity becomes higher than the predetermined value γ2, flow Fs alongthe wall of the cylinder 251 a is likely to occur, and the tumbleintensity is reduced.

FIG. 31B also shows the velocity of flow of air in the engine 200A(hereinafter referred to as “engine 200A-1”) as a comparative example inwhich the inter-stem angle θ2 is set to be larger than 6° and equal toor smaller than 10°. It is understood from FIG. 31B that, in the engine200A-1, the mainstream of intake air is drawn too much to the middle ofthe combustion chamber 254, and the flow velocity increases excessivelyto be larger than the predetermined value γ2. In the engine 200A inwhich the inter-stem angle θ2 is within the range of the aboveexpression (12), on the other hand, the intake air that flows into thecylinder can be spread out by an appropriate degree to the periphery ofthe combustion chamber 254. Thus, in the engine 200A, the width ofdistribution of the flow velocity over which the flow velocity is equalto or larger than the predetermined value β2 can be made larger than apredetermined width W2, and the mainstream of intake air is preventedfrom being concentrated too much at the middle of the combustion chamber254, so that the flow velocity becomes equal to or larger than θ2 and issmaller than the predetermined value γ2 when the valve lift of theintake valves is in a middle to high range. In the engine 200Aconstructed as described above, the intake air can be introduced intothe cylinder in a favorable manner so as to form a tumble flow T in thecylinder from the time when the valve lift of the intake valve is in asmall to middle range, and the intake air flowing into the cylinder isprevented from hitting against the wall of the cylinder 251 a at anexcessively high velocity, which would result in a reduction in theintensity of the tumble flow T formed in the cylinder.

Next, a tenth embodiment of the invention will be described. An internalcombustion engine according to the tenth embodiment is substantiallyidentical with the engine 200A of the ninth embodiment, except that whenthe intake valve 255A is in the closed state, the stem stm of the intakevalve 255A is inclined such that the distal point P11 of the stem stm islocated closer to the exhaust port 220 than the center P2 as viewed inthe direction F of flow of intake air. The intake valve 255A inclined inthis manner will be hereinafter referred to as intake valve 255B (whichrepresents intake valve 255Ba and intake valve 255Bb) In thisembodiment, the intake valve 255B is regarded as a specific intakevalve. FIG. 32, which is similar to FIG. 26, schematically shows aprincipal part of the engine 200B associated with one cylinder, asviewed in a horizontal plane on which that part of the engine 200B isprojected. FIG. 33 schematically shows the intake valve 255Bb alone,which is disposed on the right-hand side in FIG. 32 and is oriented inthe same direction as in FIG. 32.

In FIG. 32 and FIG. 33, installation angle θ3 is defined as an acuteangle formed between a straight line L6 that contains the center P2 andis substantially orthogonal to the direction F of flow of intake air,and the center axis C1 of the stem stm, as viewed in the horizontalprojection plane. By suitably setting the installation angle θ3, thestem stm may also be inclined in the direction F of flow of intake air.The arrow “POSITIVE” associated with the installation angle θ3 in FIG.33 indicates that the installation angle θ3 assumes a positive valuewhen the distal end P11 of the stem stm is located closer to the exhaustport 220 than the center P2 as viewed in the direction F of flow ofintake air.

When the stem stm of the intake valve 255B is inclined with theinstallation angle θ3 being set to 90°, it is difficult to draw theintake air toward the middle of the combustion chamber 254. Where theinstallation angle θ3 is reduced from 90°, too, a significant effectcannot be expected if the degree of the reduction is small. If theinstallation angle θ3 is smaller than 0°, on the other hand, it may bedifficult to smoothly form flow of intake air to the downstream side,right. above the umbrella portion ub. Thus, it is preferable that theinstallation angle θ3 is within in a permissible range as indicated bythe following expression (13).

0°≦θ3≦70°  (13)

If the installation angle θ is set to within the range of the aboveexpression (13), the umbrella portion ub is further inclined, and theintake air flowing along the umbrella portion ub on one side of the stemstm closer to the center of the combustion chamber 254 is also dispersedtoward the bottom dead center of the cylinder 251 a. As a result, theintake air flowing into the cylinder is more favorably prevented fromhitting against the wall of the cylinder 251 a, which would result innoticeable occurrence of flow Fs along the wall of the cylinder 251 a.Depending upon the installation angle θ3, which is in the range of theexpression (13), the dispersion of the intake air may contribute toproduction of the tumble flow T, or the intake air may be introducedinto the cylinder in a manner suitable for production of the tumble flowT. In this respect, where the tumble intensity is to be furtherimproved, it is further preferable that the installation angle θ3 iswithin a recommended range as indicated by the following expression(14).

10°≦θ3≦60°  (14)

FIG. 34A and FIG. 34B schematically show the distribution of thevelocity of flow of air measured at the middle of the combustion chamber254. FIG. 34A and FIG. 34B show the results of comparison in terms ofthe flow velocity between the engine 200B in which the installationangle θ3 is within the range of the above expression (14), and theengine 200X. More specifically, FIG. 34A shows the distribution of theflow velocity when the valve lift of the intake valve is in a small tomiddle range, and FIG. 34B shows the distribution of the flow velocitywhen the valve lift is in a middle to high range. The inter-stem angleθ2 is suitably set within the range. of the above expression (10) inaccordance with the set value of the installation angle θ3. When theinstallation angle θ3 is within the range of the above expression (14),the engine 200B provides a flow velocity that is larger than thepredetermined value α2 as shown in FIG. 34A, when the valve lift of theintake valve is in a small to middle range. The, predetermined value α2is larger than the maximum flow velocity that can be achieved by theengine 200X. When the valve lift of the intake valve is in a middle tohigh range, on the other hand, the engine 200B provides a flow velocitythat is larger than a predetermined value β2′, as shown in FIG. 34B. Thepredetermined value β2′ is larger than the predetermined value β2, andis smaller than the predetermined value γ2.

FIG. 34B also shows the velocity of flow of air with respect to theengine 200B (hereinafter referred to as “engine 200B-1”) as acomparative example in which the mainstream of intake air is drawn toomuch to the middle of the combustion chamber 254, and the flow velocityincreases excessively to be larger than the predetermined value γ2. Inthe engine 200B, on the other hand, the intake air that flows into thecylinder can be dispersed by an appropriate degree toward the peripheryand bottom of the combustion chamber 254, so that the width or range ofdistribution of the flow velocity over which the flow velocity is equalto or larger than the predetermined value β2 can be made larger than thepredetermined width W2. Thus, the mainstream of intake air is preventedfrom being concentrated too much at the middle of the combustion chamber254, so that the flow velocity becomes larger than the predeterminedvalue β2′ and smaller than the predetermined value γ2 when the valvelift of the intake valve is in a middle to high range. In FIG. 33, areaAR11 represents an area or range of installation angle θ3 correspondingto the case where the flow velocity becomes equal to or larger than thepredetermined value α2 and smaller than the predetermined value β2 inthe engine 200B, and area AR12 represents an area or range ofinstallation angle θ3 corresponding to the case where the flow velocitybecomes equal to or larger than the predetermined value β2. In thepresent embodiment, the installation angle θ3 of the intake valve 255Bis set to a value included in the area AR12.

FIG. 35A, FIG. 35B and FIG. 35C show the fuel consumption characteristicof the engine 200B during lean-burn operation, and the outputperformance during high-load operation. The fuel consumptioncharacteristic during lean-burn operation is specifically indicated inFIG. 35A by the relationship between the fuel consumption rate of theengine 200B and the air-fuel ratio, and the output performance duringhigh-load operation is specifically indicated in FIG. 35B by therelationship between the engine torque and the engine speed of theengine 200B. In FIG. 36B, the output performance is indicated by thefull-load performance. FIG. 35A and FIG. 35B show the results ofcomparison between the engine 200X and the engine 200B. FIG. 35Cquantitatively indicates the fuel consumption reduction rate and outputperformance improvement rate that are expected to be achieved when theinter-stem angle θ2 is within the ranges of the expression (11) and theexpression (12), and also indicates the fuel consumption reduction rateand output performance improvement rate that are expected to be achievedwhen the installation angle θ3 is within the ranges of the expression(13) and the expression (14).

When the stem stm is inclined by an appropriate degree, through settingof the inter-stem angle θ2 and the installation angle θ3, ahigh-intensity tumble flow T can be formed in the cylinder and strongturbulence can be produced from the time when the valve lift of theintake valve is in a small to middle range. Therefore, the combustioncharacteristics are improved during lean-burn operation, resulting in areduction of the fuel consumption rate, and the output performance canbe improved during high-load operation. As shown in FIG. 35A, the fuelconsumption rate is reduced in the engine 200B, as compared with theengine 200X, and a lean-burn region in which the engine is operable at alean air-fuel ratio is expanded. Also, as shown in FIG. 35B, the enginetorque is improved over the entire range of the engine speed in theengine 200B as compared with the engine 200X. It will be understood fromFIG. 35B that the degree of improvement of the engine torque increasesas the engine speed decreases, and that a greater effect is provided byinclining the stem stm through suitable setting of the inter-stem angleθ2 and the installation angle θ3 as the engine speed is lower.

If the inter-stem angle θ2 is set to within the ranges of the expression(11) and the expression (12), the fuel consumption reduction rate andoutput performance improvement rate as quantitatively indicated in FIG.35C are expected to be achieved. If the installation angle θ3 is set towithin the ranges of the expression (13) and the expression (14), thefuel consumption reduction rate and output performance improvement rateas quantitatively indicated in FIG. 35C are expected to be achieved. Inthe engine 200B constructed as described above, the intake air can beintroduced into the cylinder in a favorable manner so as to form atumble flow T in the cylinder from the time when the valve lift of theintake valve is in a small to middle range, and the intake air flowinginto the cylinder is prevented from hitting against the wall of thecylinder 251 a at an excessively high velocity, which would result in areduction in the intensity of the tumble flow T formed in the cylinder.

Next, an eleventh embodiment of the invention will be described. Aninternal combustion engine 200C according to the eleventh embodiment isdifferent from the engines 200A and 200B according to; the ninth andtenth embodiments in that the engine 200C has a three-intake-valvestructure, namely, each cylinder is provided with three intake valves.FIG. 36, which is similar to FIG. 26, schematically shows a principalpart of the engine 200C associated with one cylinder, as viewed in ahorizontal plane on which that part of the engine 200C is projected. Inthe engine 200C, the inter-stem angle θ formed between the stems stm ofintake valves 255Ca, 255Cb located at the opposite ends is set in amanner similar to that of the intake valves 255A. Furthermore, theinstallation angle θ3 of the intake valves 255Ca, 255Cb may be set in amanner similar to that of the intake valve 255B.

In the engine 200C, the intake valves 255Ca, 255Cb located at theopposite ends with respect to one cylinder are regarded as specificintake valves. With the specific intake valves thus provided, themainstream of intake air flowing toward the middle of the combustionchamber 254 can be increased or intensified when the valve lift of theintake valves is in a small to middle range, and occurrence of flow Fsalong the wall of the cylinder 251 a can be suppressed when the valvelift of the intake valves is in a middle to high range, even where theengine 200C has the three-intake-valve structure. In the engine 200Cconstructed as described above, the intake air can be introduced intothe cylinder in a favorable manner so as to form a tumble flow T in thecylinder from the time when the valve lift of the intake valve is in asmall to middle range, and the intake air flowing into the cylinder isprevented from hitting against the wall of the cylinder 251 a at anexcessively high velocity, which would result in a reduction in theintensity of the tumble flow T formed in the cylinder.

Next, a twelfth embodiment of the invention will be described. Aninternal combustion engine 200D according to the twelfth embodiment issubstantially identical with the engine 200A according to the ninthembodiment, except that intake ports 210D (which represent intake port210Da and intake port 210Db) are further provided with an airflowcontrol valve 260 that deflects intake air in the intake ports 210D soas to create a high-intensity tumble flow T in the cylinder. Namely, theengine 200D is equivalent to the engine 200A that is further equippedwith the airflow control valve 260. The engines 200B and 200C accordingto the tenth and eleventh embodiments may also be provided with airflowcontrol valves that operate in substantially the same manner and providesubstantially the same effect as the airflow control valve 260.

FIG. 37, which is similar to FIG. 26, schematically shows a principal.part of the engine 200D associated with one cylinder, as viewed in ahorizontal plane on which that part of the engine 200D is projected. Oneof the opposite edges of the airflow control valve 260 is pivotallysupported by a valve stem 261, and the other edge of the airflow controlvalve 260 is formed with a recessed portion K for directing intake airtoward the middle of the combustion chamber 254 when the valve 260 isclosed. To cause the recessed portion K to direct the intake air towardthe middle of the combustion chamber 254, the other edge of the airflowvalve 260 is recessed such that the depth of the recess increases as itapproaches a portion of the valve 260 corresponding to the middle of thecombustion chamber 254, and such that the recessed portion K is expandedat the center thereof. When the airflow control valve 260 is fullyclosed or half open, the intake air flowing through the intake ports210A passes the recessed portion K, thereby to be directed toward themiddle of the combustion chamber 254. By directing the intake air towardthe middle of the combustion chamber 254 before the intake air flowsinto the cylinder, the mainstream of intake air flowing toward themiddle of the combustion chamber 254 can be increased or intensified.

FIG. 38 shows the relationship between the tumble intensity and thevalve lift. FIG. 38 shows the results of comparison in terms of thetumble intensity, between an internal combustion engine 200Y and theengine 200D of this embodiment. The engine 200Y is substantiallyidentical with the engine 200D, except that the inter-stem angle θ2 isset to 0° in the engine 200Y It is understood from FIG. 38 that thetumble intensity is improved when the airflow control valve 260 is fullyclosed, rather than when the valve 260 is fully open. It is alsounderstood from FIG. 38 that in the engine 200D, as compared with theengine 200Y, the tumble intensity is improved when the airflow controlvalve 260 is fully closed from the time when the valve lift of theintake valves is in a small to middle range. In the engine 200Dconstructed as described above, the intake air can be introduced intothe cylinder in a favorable manner so as to form a tumble flow T in thecylinder from the time when the valve lift of the intake valve is in asmall to middle range, and the intake air flowing into the cylinder isprevented from hitting against the wall of the cylinder 251 a at anexcessively high velocity, which would result in a reduction in theintensity of the tumble flow T formed in the cylinder.

The illustrated embodiments are preferable embodiments of the invention.It is, however, to be understood that the invention is not limited tothese embodiments, but may be embodied with various modifications orimprovements, without departing from the principle of the invention.

1. An internal combustion engine comprising: an intake port thatcommunicates with a combustion chamber, and an intake valve that has anumbrella portion and a stem connected at one end thereof to the umbrellaportion, wherein said intake port having a starting point that is afirst point of intersection of a streamline of the intake port and aninlet-side opening plane of the intake port, and an end point that is asecond point of intersection of the streamline of the intake port and acenter axis of the intake valve, as viewed from an upper face of acylinder head in which the intake port is formed, said first and secondpoints of intersection being projected on a horizontal plane, whereinthe streamline of the intake port projected on the horizontal plane iscurved toward a center of the combustion chamber so as to be at leastpartially located closer to the center of the combustion chamber than afirst straight line that contains the starting point and extends in adirection orthogonal to an axis of a crankshaft on the horizontal plane,and a second straight line that contains the end point and extends in adirection orthogonal to the axis of the crankshaft on the horizontalplane, wherein: the intake valve comprises a specific intake valve inwhich the stem is offset such that an inner passage region locatedcloser to the center of the combustion chamber, out of two intake-airpassage regions on the opposite sides of a plane that contains a centeraxis of the stem, becomes larger, and such that the center axis of thestem does not contain a center of a bottom face of the umbrella portion.2. The internal combustion engine according to claim 1, wherein the stemof the specific intake valve is offset to an upstream side with respectto the center of the specific intake valve, in a direction of flow ofintake air.
 3. The intake combustion engine according to claim 1,wherein a portion of the umbrella portion of the specific intake valve,which corresponds to the inner passage region, has a smaller volume thana portion of the umbrella portion which corresponds to the outer passageregion.
 4. The internal combustion engine according to claim 1, whereinthe specific intake valve further includes a rotation preventing devicethat prevents the specific intake valve from rotating about the centeraxis of the stem of the specific intake valve.
 5. An internal combustionengine comprising: an intake port that communicates with a combustionchamber, and an intake valve that has an umbrella portion and a stemconnected at one end thereof to the umbrella portion, wherein saidintake port having a starting point that is a first point ofintersection of a streamline of the intake port and an inlet-sideopening plane of the intake port, and an end point that is a secondpoint of intersection of the streamline of the intake port and a centeraxis of the intake valve, as viewed from an upper face of a cylinderhead in which the intake port is formed, said first and second points ofintersection being projected on a horizontal plane, wherein thestreamline of the intake port projected on the horizontal plane iscurved toward a center of the combustion chamber so as to be at leastpartially located closer to the center of the combustion chamber than afirst straight line that contains the starting point and extends in adirection orthogonal to an axis of a crankshaft on the horizontal plane,and a second straight line that contains the end point and extends in adirection orthogonal to the axis of the crankshaft on the horizontalplane, and wherein the intake valve comprises a specific intake valve inwhich the stem is inclined, when the intake valve is in a closed state,such that a distal end of the stem is located closer to a plane thatcontains a center axis of a cylinder and is substantially orthogonal tothe axis of the crankshaft, than a center of a bottom face of theumbrella portion, in a direction substantially parallel to the axis ofthe crankshaft.
 6. The internal combustion engine according to claim 5,wherein the stem of the specific intake valve is inclined, when thespecific intake valve is in a closed state, such that the distal end ofthe stem is located closer to an exhaust port than the center in adirection of flow of intake air.
 7. The intake combustion engineaccording to claim 2, wherein a portion of the umbrella portion of thespecific intake valve, which corresponds to the inner passage region,has a smaller volume than a portion of the umbrella portion whichcorresponds to the outer passage region.
 8. The internal combustionengine according to claim 2, wherein the specific intake valve furtherincludes a rotation preventing device that prevents the specific intakevalve from rotating about the center axis of the stem of the specificintake valve.
 9. The internal combustion engine according to claim 3,wherein the specific intake valve further includes a rotation preventingdevice that prevents the specific intake valve from rotating about thecenter axis of the stem of the specific intake valve.